Alkali-treated straw and cellulose xanthogenate were derived from shoot and root biomass of Eichhornia crassipes by treatment with NaOH and CS₂, respectively. The structures of the raw and modified plant materials were characterized by XRD, TGA/DTA, and FTIR. Alkali treatment increased the crystallinity of raw plant material, while the subsequent CS₂ treatment had the reverse effect. The thermal stability of the plant material was diminished by alkali treatment but was restored by subsequent CS₂ treatment. Alkali treatment removed most of the lignin and hemicellulose from the raw plant material, whereas the formation of cellulose xanthogenate introduced new C=S and O–CS–S functional groups.